Switching power supplies are used in applications where power supply efficiency is a concern. Several different types of switching power supplies are known and commonly used. A first type is known as a "bucking" power supply. Bucking supplies are DC-to-DC converters providing a stable output voltage from an input voltage that is larger than the output voltage. In other words, by bucking a portion of the electromotive force of the input voltage, the bucking supply is able to provide a regulated, reduced output voltage. Ideally, the bucking supply is able to perform voltage step-down and regulation functions with very little power loss.
A second type of switching power supply is known as a "boosting" power supply. Boosting power supplies incorporate a transformer or coil and one or more switches coupled in series with a primary winding of the transformer or in series with the coil. Boosting power supplies are able to supply a DC output voltage from a DC input voltage that is lower than the output voltage. Examples of switching power supplies using both principles of operation are described in U.S. Pat. No. 5,691,632, entitled "Switching Power Supply," issued to Otake and hereby incorporated herein by reference. These examples use a switching transistor as a synchronous rectifier. Efficiency of switching transistor operation is improved by reducing charge storage effects.
Other examples of switching power supplies are known. For example, U.S. Pat. No. 5,675,479, issued to Tani et al. and hereby incorporated herein by reference, discloses a switching power supply where efficiency is improved under light loading by lowering the switching speed of a switching element. Under heavy load, output ripple is reduced by increasing switching speed.
U.S. Pat. No. 5,390,101, issued to Brown and hereby incorporated herein by reference, discloses a switching power supply having a voltage controlled oscillator (VCO) to provide high efficiency operation throughout a wide range of input voltages and load conditions. VCO frequency is increased when output loading increases. U.S. Pat. No. 4,683,529, issued to Bucher II and hereby incorporated herein by reference, discloses a switching power supply that uses pulse-width modulation together with switching frequency modulation to maintain high efficiency.
All of these examples are concerned with maintaining efficiency over portions of a load curve where significant amounts of power are being drawn from the switching power supply. As a result, the range of frequencies over which they operate is relatively narrow. Additionally, power dissipation in switching power supplies (i.e., inefficiency) is composed of two principal components: (i) conduction losses, caused by parasitic resistance in power supply components and (ii) switching losses, caused by charge storage and other effects in the switching elements. Switching losses are proportional to switching speed.
When the amount of electrical power being drawn from the switching power supply is reduced to very low levels, the conduction losses become very small and the switching losses are the dominant source of switching power supply inefficiency. For example, switching power supplies that have efficiencies on the order of 95% under normal loading may have efficiencies of about 50% under standby conditions, due primarily to switching losses.
Moreover, increased concern over pollution caused by power generation and increasingly larger numbers of electrically-powered devices used in homes and industry combine to create new standards and guidelines for power consumption budgets for electrical appliances. Also, increasing numbers of electrical appliances are maintained in a "ready-to-operate" state twenty-four hours a day.
Further, the EPA has developed new guidelines for compliance with Energy Star power consumption limit guidelines for power budgets for such appliances. As a result, these appliances are being designed to incorporate power-saving "standby" modes whereby the appliance is both ready to be operated and is consuming as little electricity as possible whilst in the standby mode.
Additionally, as the number and the diversity of electrically-powered appliances has increased, especially data communications, data storage and data manipulation appliances, demand has grown for battery-powered electrical appliances. Consumer trends for such appliances place heavy emphasis on size and weight for the appliances. A particular emphasis on increased battery life, and hence on reduced consumption of electrical power, places a substantial premium on reduction of power consumption in such appliances.
What is needed is a new type of switching power supply capable of extremely low power consumption in a standby mode while still being capable of supporting all required functions in a host appliance and which is also capable of switching very rapidly to full power operation on demand.